Internally hardened structure



H. E. SOM ES Feb. l, 1944.

il ZT/vigna IIIIIII IIIIIIIIIIIIIl-IIIIIIIIIIIIIIIIIIIIIII'III j* l III WIIIIIIIIIIIIIIIIIIIIl-IIIIIIIIIIIIIIIIII A Filed April 26, 1940 IIIIIIII INVENTOR 'ome Wmzx;

Patented Feb. 1, 1944 UNITED'STATES PATENT OFFICE l 2,340,349 INTERNALLY HARDENED STRUCTURE Howard E. Somes, Detroit, Mich., ,assignor to Budd Induction Heating Inc., Philadelphia, Pa., a corporation of Michigan Application April 26, 1940, Serial No. 331,733

6 Claims.

This invention relates to cylindrical structures such as pipes subjected in use to axial stresses, and engine cylinders of the air-cooled type hav- Cil harden the same by mtriding, carborizing or by some other similar process. Such processes, of substantially results ll distinct r In the case of tubular pipe, particularly pipe 4 subjected in use to axial tensile stresses, such as pipe strings havthe normal strength of the pipe at the threaded regions due to the reduce cross-section incident to the removal of metal during the thread cutting operation.

'I'he present invention is directed to novel cylindrical and tubular structures in which internal surface ,zones are provided with a hardness materially greater than that zones,

It is, therefore, an object of the invention to provide( tubular .and hollow cylindrical structures having an electro-magnetically induction hardinterna] layer and in which the normal resistance of the structure to tensile failure and to radial deformation, which means is in the form of an integral layer of met-al at the inner surface thereof zone hardened to a hardness of materially greater hardness than the outer layer and of such quality of hardness as to increase the yield strength of the structure.

`A yfurther object is to provide such a tubular structure in which the internal zone-hardened layer of metal is of such increased yield strength as to compensate at least in part for the loss in strength occasioned by the removal of surface metal in external machining of the same.

The above and other novel features of the invention will appear more fully hereinafter from the following detailed description when taken in conjunction with the accompanying drawing. It

is expressly understood, however, that the drawing is employed for purposes of illustration only and is not designed as a definition of the limitation of the invention, reference being had for this purpose to the appended claims.

In the drawing wherein like reference characters indicate like parts,

Figure 1 illustrates a section through a cylinder having substantially {the configuration required by air-cooled internal combustion engines.

Figure 2 illustrates a ystep in the process of forming such a cylinder from a blank of uniform annular cross-section throughout its length.

Figure 3 illustrates a pipe structure in which the process has application.

Referring to Figure 2 there is illustrated in section a cylindrical .blank which may be a forging or casting from which an engine cylinder is to be formed. The blank is machined to a uniform bore and preferably to a uniform wall thickness and the internal bore is then hardened to a depth of approximately a tenth vof an inch or less by electromagnetic induction heat treating apparatus illustrated, the apparatus comprising a coil l0, preferably a flux concentrating core I2 and an associated annular quench I4 adapted to quench the heated zone almost instantaneously after heating progressively. The apparatus which may be employed is similar to that disclosed in a copending application Serial No. 277,996, led June 8, 1939, now Patent No. 2,281,333, issued April 28, 1942, and the details thereof need not be described further.

The cylinder blank for air cooled engines must be composed of a material which, when hardened, will not lose its hardness through a range of temperatures up as high as 550 F. Such a material `as that constituted by S. A. E. 4140 has such properties. Also a material similar thereto but in which tle carbon content is increased to about .50%, the chromium content reduced to about .55% and the molybdenum increased to .75% with as much as 1% of silicon will be found to withstand the Working temperatures of such cylinders. The blank, after having preferably been machined to a proper external cylindrical and internal bore dimension, is hardened internally to a uniform depth and thereafter may be drawn for a period of time at a temperature substantially the maximum to which the cylinder may be subjected in use. During the hardening operation, the thick wall of the blank maintains the cylinder against any possibility of distortion or warping. The heated layer is raised to a plastic state and consequently when it cools from quenching will be forced to adhere to its original shape because of the backing metal which has not been affected by the heat treatment.

ened wall or to any After hardening the inner/surface zone of the blank, external cooling fins and mounting flanges may be formed by machining grooves in the exterior of the blank. It will be found that because of the controlled depth of hardening, the grooves cut for the purpose of forming the cooling fins may be cut substantially to the harddegree less without danger of running into even slightly hardened material. This is important in the cutting of deep narrow grooves such as are required to effect the formation of a great many closely spaced cooling fins for if there were danger Yduring the machining operation of the cutting tools striking a hard spot deep down within a groove between adjacent cooling fins, fracture of the cooling fins during the machining operation would almost surely result.

As is illustrated in Figure 2, the blank i6 is provided with a hardened internal layer I8, and referring to Figure 1, grooves 20 are turned in the external unhardened portion of the blank after hardening of the internal surface zone in order to form closely spaced fins 22 and such end attachment threaded portions as 24 or anges as at 26 as may be desirable.

l the pipe which Where threaded ends are desired, a peculiarityin the structure resulting from the hardening operation described permits of unusual lightness at this point, and this is because the internal layer of hardened material is under a residual trapped compressive stress, and due to its hardness has a greatly increased yield strength. This greatly increased yield strength renders it unnecessary to increase the thickness of the tube to compensate for the loss of strength that would usually appear to be necessary as a result of the weakening effect of the thread grooves, and yet the hardened layer is so restricted as to leave' the remainder readily ymachinable as in thread cutting.

In a threaded pipe end for example, and where the pipe is subjected to axial stresses, such as in thin-walled pipe sections connected together by internally threaded couplings, the weakening effect of the threaded end can be offset by hardening an internal annular layer of the metal of is of such longitudinal extent as to extend under the threaded portion, just as in the case of the cylinder previously described. In Figure 3, this structure is illustrated. The pipe end 40 is provided with external threads 42, which in effect reduce the cross-section of the v pipe and consequent strength. Through the internally hardened layer 44, which has a correspondingly increased yield strength, the yield strength of the pipe `end despite the reduced cross-section is the same as the rest of the pipe and without the necessity of enlarging the end. Thus, the integral zone hardened layer 44 of metal, vspaced from the roots of the threads 42 and extending longitudinally of the pipe within the threaded region, provides, due to its increased yield strength, a means for increasing the normal resistance of the pipe to tensile failure and to radial deformation and collapse. whichis of particular advantage at the threaded `region inasmuch as the increased resistance to radial deformation due to the increased yield strength decreases the tendency of the pipe threads to pull out of the connecting couplings or other member during the presence of heavy axial tensile stresses. Although the inner zone hardened layer of metal is shown in Figure 3 as terminating near the innermost end thread in which internal layer of metal, both in the case of the the yield strength of the threaded region oi.' the cylinder and the pipe, should be that which will pipe may be increased at least to that of the give the most emcient results in use. In the threaded region. The threads mayI be just as on the amount of increased yield strength deeasily cut after the hardening, as ln a plain unsired to be imparted to the tubular structure in hardened pipe, for the zone in which the threads question. l

are cutis unchanged by the hardening of the in- Although the invention has been described in scribed may have a hardened internal layer of Suggested in enjllnetion with those materials as sides thereof the internal bore being cooled by e had e appended claims for a definition the quenching iluid and the inner transition zone of the limits of the invention.

between the hardened layer and the nnhardened at is claimed portion being cooled by the mere exchange` oi' 1 A tubular structural member comprising a tubular member to be internally hardened be 0f its internal diameter, and havingits wall head through the length of the bore, while inhe creation of axial stresses arising from such trolling the end eifect at either end oi' the tubu 40 length 0f Said Wall portion, which hardness is of extensions is illustrated in the patent above quality characterized by rapid electromagnetic referred te, induction heating and substantially instantane After the internal hardening operation and the 011s qnenehng, the metal 0f Said outer layer being external machining operations are completed, 0f Sllh 10W hardness as t0 be readily machinable the internal bore, in the case of the cylinder, by machine Cutting operations and retaining subof said tubular portion.

e 2'. A cylindrical tubular metal structure having ployed, the initial hardness in the internal laythe metal man annular Zone adjacent the inner er may be Rockwell C 60 and after drawing Vthe surface thereof for a portion at least of its length scale than the metal of the zone surrounding it,

Derature of 1560 F. and after extended use at a 'hardness 0f the metal 0f Said inner 20H8 being' temperature of 550 F. or after a prolonged draw that Which iS characterized by rapid electromagat this temperature will still retain a Rockwell neC induction heating and substantially instanhardness 0f about; 57 (1 60 taneous quenching and the radial depth of said Thus it will appear that exceedingly tough a]- inner zone being such that the yield strength of loys may be employed for making up cylinder said portion at said region'of reduced cross-secstructure, which alloys may be hardened intertion is equal at least to that which would exist nally without rendering the external metal too in Said region in the absence 0f hardening 'of said hard to machine. As' a result, the hardening 6" inner zone and reduction in cross-section of said operation may be effected at a time when the surrounding zone, whereby to increase the nonl on. and outer annularlayers of metal of the same It is to be understood that the hardness of the 0 composition throughout extending substantially from end to end thereof, said outer layer having the metal of an annular layer of substantially machined portions therein resulting in anon-unipredetermined radial depth originating at the form radial thickness of said outer layer, said inner wall of a hardness different from that of hardness which is substanthe metal surrounding it, said surrounding metal tialiy uniform throughout its radial depth and 5 within said portion being of relatively low hardmaterially greater thanv that of said outer 'layer ness and having external machined` portions as measured on the Rockwell C scale and being therein which effect areduction incrosls-sectional of such radial thickness and having such quality area and a consequent reduction in strength of of hardness as to constitute a means for increassaid first portion throughout its machined region, ing the yield strength of said inner layel` an the metal comprising said annular layer being of amount sufficient to increase the strength of said such greater hardness than that of said surstructure as a whole while compensating for the rounding metal and said annular layer being of loss in strength occasioned by the production of such radial depth as to increase the normal said machined portions. strength of said rst portion as machined an 4. A cylindrical tubular metal structure having amount sufcient to substantially offset said reexteror end threads and an annular layer of duction in strength. the met-al of its inner wall of a hardness and of y6. In a tubular structure having an exteriorly a yield strength materially greater than the hardthreaded end portion and which is to be subness and yield'strength, respectively, of the rejected to axial stresses in use, means for commainder of the metal surrounding it, said threads pensating for the-loss in strength of said strucbeing in said surrounding metal and the length of ture due to the presence of the threads therein, said layer of greater hardness and yield strength comprising an integral layer of metal adjacent being different from that of said threads, said the inner wall thereof and extending longitudivlayer of metal of greater hardness and yield nally a substantial distance beyond said threads strength constituting a means for compensating 2 and being suiciently harder than the thread for lthe weakening effect of the threads in Said carrying metal to increase the yield strength of Surrounding metal on said structure and for insaid structure at said threaded end portion to at creasing the normal strength of said structure at least substantially that of the remainder of said the region of said threads. structure. 5. A one-piece tubular metal structure having l HOWARD E. SOMES.

for a portion at least of its length from one end 

